How long to solve this puzzle?

What is the number in the space that the car is obscuring?

This was part of test at a Hong Kong elementary school.  The children were expected to solved it in 20 seconds.

It’s one of those puzzles that – apparently – children solve much quicker than adults.  Loads of smart-alecks on the web are boasting that they got it in under ten seconds.  It took me longer than that.  (But loads of my friends didn’t get it at all, heh he!)

The answer is in the post below.